As is well known, fluid dynamic bearing devices have features in their high speed rotation, high rotational accuracy, quietness, and the like. Through a good use of such features, the fluid dynamic bearing devices are suitably used as bearing devices for motors to be mounted to various electrical apparatus such as information apparatus, and more specifically, as bearing devices for spindle motors to be built in disk drives of HDDs and the like, or bearing devices for fan motors to be built in PCs and the like.
The fluid dynamic bearing device includes a radial bearing portion for supporting a rotary body with respect to a stationary body in a radial direction and a thrust bearing portion for supporting the rotary body with respect to the stationary body in thrust directions. In recent years, each of both the radial bearing portion and the thrust bearing portion has been formed of a fluid dynamic bearing that supports the rotary body in a non-contact manner in many cases.
There have been proposed various fluid dynamic bearing devices including the radial bearing portion and the thrust bearing portion, each being formed of the fluid dynamic bearing. For example, in FIG. 1 and FIG. 6 of Patent Literature 1, there is illustrated a fluid dynamic bearing device including a radial bearing gap in the radial bearing portion, which is formed by an inner peripheral surface of a sleeve portion (referred to as “fluid dynamic bearing” in Patent Literature 1) made of a sintered metal arranged on the stationary body, and a thrust bearing gap in the thrust bearing portion formed by an end surface of the sleeve portion on one side in an axial direction. When the thrust bearing portion is formed of the fluid dynamic bearing, a thrust dynamic pressure generating portion is formed on any one of two opposed surfaces that form the thrust bearing gap. The thrust dynamic pressure generating portion generally includes a plurality of dynamic pressure generating grooves arranged in a herringbone pattern or a spiral pattern and convex hill portions that define and form the dynamic pressure generating grooves. In this case, along with rotation of the rotary body, lubricating oil in the thrust bearing gap flows along the thrust dynamic pressure generating portion. The lubricating oil is pumped into a narrow-width portion of the thrust bearing gap, which has a smaller gap width, to increase a pressure of the lubricating oil in the thrust bearing gap (a dynamic pressure generating action is caused in the lubricating oil present in the thrust bearing gap). As a result, the thrust bearing portion formed of the fluid dynamic bearing is formed.
Incidentally, when spaces are formed on both sides of the sleeve portion in the axial direction as in the case of the fluid dynamic bearing device disclosed in Patent Literature 1, pressure balance of the lubricating oil filling the two spaces is lost along with an operation of the bearing device in some cases. If the thus lost pressure balance is left, support accuracy for the rotary body (bearing performance) in the thrust directions is disturbed.
In order to prevent the occurrence of the problem described above as much as possible, also as described in Patent Literature 1, it is effective to form a communication path for bringing both end surfaces of the sleeve portion (the space formed by one end surface of the sleeve portion and the space formed by another end surface of the sleeve portion) into communication with each other. Specifically, if the communication path as described above is formed, even when the pressure balance of the lubricating oil between the above-mentioned two spaces is lost, the lubricating oil flows from a higher pressure side to a lower pressure side through the communication path to restore the lost pressure balance at an early time. Therefore, desired bearing performance (bearing performance in the thrust directions, in particular) can be stably maintained. In the configuration of Patent Literature 1, the communication path can be formed by forming axial grooves on an outer peripheral surface of the sleeve portion or an inner peripheral surface of a housing opposed thereto.